Vehicles may include cooling systems configured to reduce overheating of an engine by transferring the heat to ambient air. Therein, coolant is circulated through the engine block to remove heat from the hot engine, and the heated coolant is then circulated through a radiator near the front of the vehicle. Heated coolant may also be circulated through a heat exchanger (e.g., heater core) to heat a passenger compartment. The cooling system may include various components such as various valves and thermostats. As such, the various components may need to be periodically diagnosed to verify functionality.
One example approach for identifying thermostat degradation is shown by Niki et al. in U.S. Pat. No. 6,240,774. Therein, an actual engine coolant warm-up profile is compared to an expected coolant warm-up profile, and thermostat valve degradation is identified based on discrepancies between the two profiles. The expected warm-up profile compensates for heat losses incurred due to vehicle speed, ambient conditions, engine load, etc. If the expected coolant temperature reaches a reference value before the actual coolant temperature, thermostat degradation is determined. In addition, if the actual temperature is much higher than the expected value, thermostat degradation may be determined.
However, the inventors herein have identified potential issues with such an approach. As an example, in engine cooling systems where various valves may be adjusted to maintain different coolant temperatures in different regions of the coolant line, the coolant warm-up profile may also be affected by the state of the various valves. Specifically, based on the state of the various valves, a portion of coolant exposed to the thermostat may vary. Further, based on a source of the coolant circulating at the thermostat (e.g., from engine, from heater core, etc.), a coolant temperature affected at the thermostat may vary. Thus, during a condition when the thermostat is exposed to cooler coolant, the actual warm-up profile may be shallower than expected and a false positive indication of degradation may occur. Likewise, during a condition when the thermostat is exposed to warmer coolant, the actual warm-up profile may reach temperatures that are much higher than expected, and a false positive indication of degradation may occur.
Thus in one example, some of the above issues may be at least partly addressed by a diagnostic method comprising, adjusting a state of one or more cooling system valves to stagnate a first amount of coolant in a first loop while exposing a second amount of coolant to a thermostat in a second loop; and indicating thermostat degradation based on a difference between a coolant temperature and a threshold, the threshold based on the state of the valves. In this way, cooling system thermostat degradation may be determined based on various thermal differentials created in different regions of the cooling system.
For example, a cooling system may be configured to circulate coolant to various vehicle system components via a plurality of valves (including a bypass shut-off valve, a heater shut-off valve, a thermostat valve, a transmission cooling valve, a transmission heating valve, etc.). During an engine cold start, the bypass shut-off valve and the heater shut-off valve may be closed to stagnate coolant at the engine, thereby expediting engine warm-up. Then, the bypass shut-off valve may be opened while a position of the remaining valves is adjusted based on vehicle cabin heating demands, and transmission heating/cooling requirements. As such, once the bypass shut-off valve is opened, the heated coolant from the bypass loop may start circulating at the cooling system thermostat. Likewise, based on the position of the remaining valves, an amount of coolant as well as a temperature of coolant circulating at the cooling system may vary. Thus, based on the state of the various valves and the engine operating conditions, a threshold (or expected) coolant temperature (or expected coolant warm-up profile) may be determined and compared to an actual coolant temperature (or actual warm-up profile) at the thermostat. Then, based on differences between the estimated/threshold values and the actual coolant temperature values, thermostat degradation may be determined. For example, if the actual coolant temperature is much higher than the expected/threshold value, based on the current state of the various valves, it may be determined that the thermostat valve is stuck open. According, engine operation adjustments may be made to compensate for the degraded thermostat valve.
In this way, by adjusting the threshold temperature (against which the estimated coolant values are referenced for diagnostic purposes) based on the state of the various valves in the cooling system, a more accurate value of the expected coolant temperature may be determined, reducing the likelihood of false positive determination of thermostat degradation. In addition, by running a thermal management diagnostic to detect the thermal state of a system not intended to receive warm coolant rather than detecting a thermal state of a system expected to receive warm coolant, the sensitivity of the diagnostic method may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.